Participants at a workshop on celebrating 50 years of pseudopotentials and their impact on the field of condensed matter physics.

The Institute for the Theory of Advance Materials in Information Technology (ITAMIT) sponsored a workshop on the influence that the pseudopotential concept has had on condensed matter physics. The workshop was held on April 11-12 in Austin, Texas. It was attended by over 50 participants, including international participants from Germany, France, Portugal, Spain, Israel, Brazil and Hong Kong. The pseudopotential concept allows one to eliminate chemically inactive electron states from the electronic structure problem. Within a pseudopotential approach length and energy scales are set only by the valence electron states. This enormous simplification allows one to consider problems of unprecedented complexity such as performing molecular dynamical simulations with quantum forces or to examine phase stability as a function of pressure and temperature.

Institute for the Theory of Advanced Materials in Information Technology: James R. Chelikowsky (University of Texas at Austin), DMR 0551195

Yousef Saad and Renata Wentzcovitch (Minnesota), Steven Louie (UC Berkeley) and Efthimios Kaxiras (Harvard): (Celebrating 50 Years of Pseudopotentials)

Variation in energy and geometry during the diffusion of Mn in CdSe

Institute for the Theory of Advanced Materials in Information Technology: James R. Chelikowsky (University of Texas at Austin), DMR 0551195

Yousef Saad and Renata Wentzcovitch (Minnesota), Steven Louie (UC Berkeley) and Efthimios Kaxiras (Harvard): Size effects on diffusion barriers in semiconductor nanocrystals

An extension to contemporary electronics can be made by exploiting both the charge and the quantum spin of the charge carriers in semiconductors. This can be achieved by doping the semiconductor with magnetic impurities. However, the effect of confinement can strongly influence our ability to dope nanocrystals. We examined the diffusion of Mn within bulk CdSe crystals and within CdSe nanocrystals. The energy barrier to diffuse within a nanocrystal is 2 to 3 times smaller than that in the bulk. As such, the impurity atom can diffuse much faster in a nanocrystal. If the dopant diffuses to the surface of a nanocrystal, it may “self-purify” the nanocrystal. This effect may account for the difficulty in doping nanocrystals.

Saddle point

The size dependence of the dopant wave function and hyperfine splitting for P-doped Si nanocrystals

Electron spin resonance can be used to study the electronic properties of doped semiconductor nanocrystals. In particular, such experiments can measure the hyperfine splitting of the dopant wave function, which probes how localized the wave function is on the dopant. We examined the evolution of hyperfine splitting for P-doped Si nanocrystals with size. Owing to quantum confinement, the dopant wave function is confined in a smaller and smaller volume as the nanocrystal size decreases, leading to an increase in hyperfine splitting. Apart from predicting the trend, our calculated results can accurately reproduce the experimental data for large Si nanocrystals.

Institute for the Theory of Advanced Materials in Information Technology: James R. Chelikowsky (University of Texas at Austin), DMR 0551195

Yousef Saad and Renata Wentzcovitch (Minnesota), Steven Louie (UC Berkeley) and Efthimios Kaxiras (Harvard): Size limits on doping phosphorus into silicon nanocrystals

Ferropericlase, (Mg(1-x),Fex)O, B1 structure, is a solid solution stable to multi-Mbar pressures. Under pressure, ferrous iron undergoes a high-to-low (S=2 to S=0) spin transition between 45 and 65 GPa at 300 K. This type of transition is a general phenomenon in transition metal compounds, can be explained by several arguments, but is most simply understood by enthalpic arguments: iron in the LS state is smaller and therefore more stable than in the HS state under compression. We have developed a thermodynamics model to quantify this spin “crossover” phenomenon [1]. The fraction of LS states versus pressure (P) and temperature (T) was predicted based on first principles LSDA+U results [1] and confirmed experimentally [2]. With this, new questions emerged concerning the physical properties of spin-crossover materials such as their elasticity [3] and conductivity.To tackle this problem we developed a vibrational virtual crystal model to investigate with predictive accuracy the thermodynamics properties of the mixed spin (MS) state. We demonstrated that there is an anomalous softening of the elastic moduli, including the bulk modulus, that is associate with the enhanced thermal expansivity of the MS state [4].

[1] Tsuchiya et al., Phys. Rev. Lett.96, 198501 (2006)[2] Lin et al., Science 317, 1740 (2007).[3] Crowhurst et al., Science 319, 451 (2007).[4] Justo et al., Nature, under review (2008).

Figure 1 (P,T) color map of the fraction of LS irons along the spin crossover transition in ferropericlase with xFe = 0.1875 [4]. Black and white lines correspond to n(P,T) = 0.5 with and without inclusion of vibrational free energy, respectively. Plus symbols are experimental data [2] for n = 0.5.

Institute for the Theory of Advanced Materials in Information Technology: James R. Chelikowsky (University of Texas at Austin), DMR 0551195

Yousef Saad and Renata Wentzcovitch (Minnesota), Steven Louie (UC Berkeley) and Efthimios Kaxiras (Harvard): Anomalous elasticity along spin crossover transitions (Intellectual

merit)

1) Some materials of interest to spintronics for their colossal magnetoresistance, such La(1-x)SrxCoO3, have end-members that undergo spin transition at ambient pressure upon heating. Our research reveals a broader picture of this phenomenon that is equally applicable to end-member LaCoO3. The spin transition in the latter has been a topic of some debate for a couple of decades.

2) Minerals of the Earth’s mantle such as Mg(1-x)FexO, Mg(1-x)FexSiO3

perovskite and post-perovskite, undergo spin crossover transitions at mantle conditions. The elastic anomalies induced by this transition should have dramatic consequences for the interpretation of seismic tomographic data. Interpretation of this data relies on the existence of thermoelastic data on minerals provided by experiments or theory. Currently elasticity experiments are not possible in these minerals at relevant conditions (T> 2000 K, P>45 GPa). This theory is therefore necessary for progress in our understanding of mineral physics. 3) This research interfaces with developments in the forefront of electronic structure methods for strongly correlated systems, such as the DFT+U. These materials are still very challenging for first principles theory and our investigations are pushing developments in this area.

4) The education of student and post-docs has been an important outcome of this grant. Several post-docs have been partially supported by this grant throghout the years: Taku Tsuchiya, João Francisco Justo, Zhongqing Wu, and Koichiro Umemoto. Two of them have accepted academic positions (TT, Ehime U. (Japan) and JFJ, U. of São Paulo (Brazil)) while the others remain highly productive in our group.

Figure 2 Pressure dependence of the calculated (a) adiabatic bulk modulus, KS, and (b) bulk wave velocity, Vφ and density, ρ, of Mg1-xFexO (xFe = 0.1875) along several isotherms [1]. Full (dashed) lines are results within (outside) the (P,T) regime of validity of the quasiharmonic approximation (QHA). Crosses are experimental results [2].

[1] Justo et al., Nature, under review (2008)[2] Lin et al., Nature 436, 377 (2005)

Institute for the Theory of Advanced Materials in Information Technology: James R. Chelikowsky (University of Texas at Austin), DMR 0551195

Yousef Saad and Renata Wentzcovitch (Minnesota), Steven Louie (UC Berkeley) and Efthimios Kaxiras (Harvard): Anomalous elasticity along spin crossover transitions (Broader

impact)

Institute for the Theory of Advanced Materials in Information Technology: James R. Chelikowsky (University of Texas at Austin), DMR 0551195

Yousef Saad and Renata Wentzcovitch (Minnesota), Steven Louie (UC Berkeley) and Efthimios Kaxiras (Harvard): Evolution of Magnetic Structures from the Atom to the Bulk

One of the most intriguing issues in condensed matter physics is the emergence of properties from the atomistic to crystalline phase. In this work, we examined how the structure of iron crystals evolve from the atom to the bulk phase. We considered iron clusters containing up to 600 atoms and examined how the the energies of clusters in different structures compared to bulk iron, which crystallizes in the body centered cubic (bcc) phase. We determined that above about 150 atoms, the bcc phase starts to become the most stable form for clusters. Examining clusters of this size required the innovation of new algorithms.

Total energy of optimized iron nanoclusterswith complete atomic shells. The lines are only guides to theeye. Reference is the isomer with bcc structure which marks theground state for bulk iron. The shellwise Mackay-transformedstructure (circles) is preferred to the icosahedral (triangles) andcuboctahedral (diamonds) structures. Shaded circles markMackay-transformed clusters with different magnetic structures.For comparison, the energy of fcc bulk iron is included. Theinset shows the size dependence of the total energies of variousgeometries evaluated at the experimental bcc lattice constant.Reference is the energy of bulk bcc iron. Above 147 atoms, thebcc isomers are systematically lower in energy than all others

Understanding electronic and optical properties of organic molecular crystals is crucial for advancing the field of “plastic” electronics. Among all organic single crystal field effect transistors, rubrene (C42H28) based FETs demonstrates the highest mobility. To understand the optical spectroscopy and electronic excitonic effects in rubrene single crystal, we examined the optical excitations using pseudopotentials-density functional theory and theories beyond including GW approximation, Bethe-Salpeter equations, and Quantum Monte-Carlo method. The figure shows the real and imaginary part of the refractive index along the three crystalline axes. The theoretical absorption spectrum agrees very well with the ellipsometry data. We demonstrated that many body effects dominate the optical spectrum and quasi-particle gap of molecular crystals. We were able to interpret the yellow-green photoluminescence observed as a result of inter-molecular charge transfer spin singlet excitons, and predicted the character of phosphorescence excitation at the red end of the optical spectrum.

Institute for the Theory of Advanced Materials in Information Technology: James R. Chelikowsky (University of Texas at Austin), DMR 0551195 Yousef

Saad and Renata Wentzcovitch (Minnesota), Steven Louie (UC Berkeley) and Efthimios Kaxiras (Harvard): Electronic and Optical Excitations in Organic Molecular Crystals

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b

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Institute for the Theory of Advanced Materials in Information Technology: James R. Chelikowsky (University of Texas at Austin), DMR 0551195

Yousef Saad and Renata Wentzcovitch (Minnesota), Steven Louie (UC Berkeley) and Efthimios Kaxiras (Harvard): Switchable Ni-Mn-Ga Heusler nanocrystals.

Heusler alloys are sometimes addressed as “crystals which replace machines” or “crystals which remember their shape”. They are members of a relatively small family, called “shape – memory materials”. These materials undergo a reversible structural transformation, which can be controlled by external parameters, like temperature, pressure or magnetic field. The structural changes lead to macroscopic changes in the shape of the crystal that can be used to activate various processes. Consequently, these materials are sometimes called actuators. One could imagine that such actuators could work on the nano scale as well, and change properties of nano structures. For example, a nano actuator coupled to a carbon nanotube can bend the nanotube and change its transport properties. Many other applications can be made. First principles calculations have shown that Heusler nano particles can be actuators. Figure (Top) shows a schematic view of a transformation. Figure (Bottom) shows how the ability to transform develops in Ni-Mn-Ga as a function of the size.

Institute for the Theory of Advanced Materials in Information Technology: James R. Chelikowsky (University of Texas at Austin), DMR 0551195

Yousef Saad and Renata Wentzcovitch (Minnesota), Steven Louie (UC Berkeley) and Efthimios Kaxiras (Harvard): Vibrational dynamics of nanocrystals.

The vibrational propertes of nanocrystals provide a wealth of information about the structural properties of these systems. Experimental methods can access these properties by utilizing Raman or Infrared spectroscopy techniques. Unfortunately, theoretical studies of the vibrations in nanostructures require large computational resources. Efficient calculations are possible if one uses new algorithms, which allows us to calculate accurately and rapidly configurations for slightly displaced atoms. These algorithms involve real space pseudopotentials and subspace filtering to solve for the quantum mechanical forces. The figure (top) compares vibrational spectra of two Si nanocrystals. These two nanocrystals are constructed in different ways: one has an atom at the origin (center), while the other has an empty interstitial site at the origin. Also shown in the figure (bottom) is a CdSe cluster doped with a Mn atom. One can see directly the contribution of the dopant in the vibrational spectra. Such contribution is position dependent and should help to determine the location of impurities in nanocrystals from experimental data.